Is it better to sleep on your back, left side or right side and why?

Makes no difference. There is no "better" way to sleep for most people. What's important is that you're comfortable and sleep well. It may be better to put a pillow between your knees if sleep on side so that you're hips and back don't get twisted which could potentially lead to back pain.
Back. Most agree the back is best. You have less pressure on the limbs and hips and joints in this position. Back and neck are supported in a straight line, diaphram is not compressed, and no lateral stress on lungs or heart. Get your body in a relaxed position, proper head support, and give it a try. Good luck.